Clamp-on flow meters are widely used in numerous industrial sectors. One of their major advantages is the fact that the flow measurement takes place without any contact with the flowing fluid.
The acoustic transducers used for the clamp-on flow measurement consist of a transducer wedge and a thereon mounted electromechanical acoustic transducer element, hereinafter referred to as transducer element, which is generally realized using a piezoceramic element. The acoustic transducers are mounted on the outside of the pipe in which the flow rate is to be measured. Hereinafter, the pipe is referred to as measuring pipe. The acoustic transducers are positioned in such way that an ultrasonic signal can be sent through the measuring pipe by one acoustic transducer to the other acoustic transducer and the acoustic beam in the flowing fluid has an angle smaller than 90 in relation to the pipe axis. Clamp-on flow meters according to the transit-time difference method measure the difference of the transit times of the two acoustic signals that propagate in and against the flow direction and use the transit time difference to calculate the volumetric flow rate. The relationship between the measured transit times and the flow velocity is described, for example, in WO 8808516 A1. The line-averaged flow velocity Vl along the sound path can be calculated from the transit time difference Δt and the transit time t1 in the fluid according to the following equation:Vl=Ka*(Δt/2t1)  Eq. (1)
Therein, Ka is the transducer constant that determines the angle of incidence in the fluid:Ka=c_alpha/sin(alpha)  Eq. (2)
Here, alpha and c_alpha correspond to the angle of incidence and the sound speed in the transducer wedge, respectively. In order to calculate the volumetric flow rate, the fluidmechanical calibration factor KF, which describes the ratio of the area-averaged flow velocity and the line-averaged flow velocity along the sound path, has to be known:KF=VA/Vl  Eq. (3)
Therefore, the volumetric flow rate Q is calculated from the cross-section area A of the pipe asQ=KF*A*Ka*(Δt/2t1)  Eq. (4)
One advantageous embodiment of the ultrasonic clamp-on flow measurement is described, for example, in DE19808642. Due to the design of the cross-section area of the pipe, the fluidmechanical calibration factor KF is designed in such way that it is independent of the condition of the flow. In DE10312034B3, an ultrasonic flow measurement method is described which allows a particularly precise determination of the fluid transit time t1 by measuring consecutive ultrasonic signals which pass through the measuring pipe multiple times.
EP0733885A1 describes a method for ultrasonic clamp-on flowmeters of compensating for the pressure and temperature dependence of the fluid by determining the sound speed of the fluid and adjusting the sound path within the fluid according to the law of refraction. The sound speeds of the transducer wedge and the pipe wall with their temperature dependences are assumed as known.
DE102009046871A1 describes a method of calibrating the transmitter of an ultrasonic clamp-on flowmeter. This allows a calibration of the time measurement required according to Eq. 1 which is independent of the acoustic properties of the measuring pipe and the acoustic transducers. Using this method, the transducer constant Ka is assumed as known and invariable.
In principle, it is assumed that the angle of incidence in the fluid is determined according to the law of refraction from the transducer constant Ka and the sound speed in the fluid. However, the wall of the measuring pipe can lead to a non-negligible deviation from sound propagation according to the law of refraction. Measurements have shown that, in that case, the transducer constant Ka used in Eq. (1) does not exactly represent the ratio of the flow velocity Vl and the transit time difference Δt as well as the transit time t1 instead of the transducer constant Ka calculated from the parameters of the acoustic transducer in Eq. (2), it would be necessary to use a factor containing the influence of the pipe wall in Eq. (1). This factor could generally be called acoustic calibration factor. Ideally, when there is no influence of the pipe wall, it would be identical with Ka; usually, however, it deviates more or less strongly from Ka. Because the pipe wall does not influence the sound speed in the transducer wedge, this deviation can only be interpreted as a change of the angle of incidence alpha.
A major advantage of the clamp-on flow measurement is the fact that the acoustic transducers can be installed on a pipe already present at the measuring point. If this advantage is to be utilized, the flowmeter cannot be calibrated at the manufacturing site together with the measuring pipe. Therefore, a possible influence of the measuring pipe on the acoustic calibration factor has to be compensated for after the installation of the acoustic transducers at the measuring point on the measuring pipe. For this, it is necessary to quantify this influence, i.e., to determine the acoustic calibration factor mentioned previously. The determination of a calibration factor for a flowmeter which is already located at the measuring point is called field calibration. In the process, the value displayed by the flowmeter is compared with the value displayed by a reference flowmeter. Usually, however, there is no reference flowmeter present at the measuring point. Therefore, it is preferable to determine the acoustic calibration factor without relying on a reference flowmeter.
The method described in DE 102004031274 B4 is, in principle, suited for this objective. However, due to the necessary mutual translation of the acoustic transducers, this method requires substantial effort if the method is to be applied at the measuring point instead of in a calibration laboratory.
DE 10221771 A1 shows an acoustic transducer for an ultrasonic flowmeter with multiple piezo elements which are combined to form a piezo array. Generally, an array is a number of transducer elements arranged in a plane, where the transducer elements can be triggered independently and, when combined, also form a transducer element. The transducer elements which form the array are called array elements. In the case of a piezo array, the array elements are piezo elements. Therefore, it is possible, by using an acoustic transducer attached flatly to the measuring pipe wall, to achieve different angles of incidence into the measured fluid for the ultrasonic signal with one wave front in relation to the measuring pipe axis. However, delayed triggering requires a lot of computational power. Furthermore, the change of angle is only possible within a limited range. If the angle of incidence of the ultrasonic signal is very flat, longitudinal waves can be excited, which can lead to a decrease in the transmission through the pipe wall and to the reflection of a significant part of the sound wave.
DE 102008029772 A1 describes a measuring system and a method for determining and/or monitoring the flow rate of a measured fluid in a measuring pipe using a first acoustic transducer and at least a second acoustic transducer. The second acoustic transducer is equipped with at least two transducer elements. The signals obtained during a diagnosis phase are used to select the transducer elements of the second acoustic transducer to be used during the measurement phase. By doing this, it is possible, for example, to reduce the effect of a sound speed change that might occur after the installation. A field calibration is not possible with this method.